Father. I truly believe that there are many Catholics who do not know the correct answer to the following question:
Who was the biological father of Jesus?
The answer is: God, through the Holy Spirit.
Knowing that, then they can understand how Jesus is the Son of God.

Answer by Fr. John Echert on 12/12/2012:

I do have a problem with using the word "biological" since that regards physical, natural generation. How about this: the offspring of a parent is of the same nature or species. A man produces a human being, a monkey a monkey, a dog a dog, a cat a cat. By analogy, since Jesus is the Son of God--eternally begotten of the Father--it follows that He is of the divine nature as well. You are probably correct, however, that many Catholics do not get it, having been poorly instructed or never instructed.